1. Field of the Invention
This invention relates to a pneumatic radial tire for passenger car use, and more particularly to an improvement of a belt in a low-profile radial tire suitable for high-speed running.
2. Related Art Statement
Recently, passenger cars can stably be run at super-high speeds of 200-300 km/h together with technical innovation, and consequently there are developed low-profile radial tires having sufficient running performances even at such a super-high speed and an aspect ratio of, for example, not more than 0.6.
When the tire is run at a super-high speed, there are particularly caused problems such as separation between tread and belt, chipping of tread rubber and the like. In order to solve these problems, it is important for the tire to be high in the durability or so-called high-speed durability.
As a tire structure considering such a high-speed durability, there is known a structure in which a belt is arranged outside a carcass. This belt is comprised of at least one main belt layer arranged outside the carcass and an auxiliary belt layer containing rubberized organic fiber cords wound outside and around the main belt layer under a certain tension.
Particularly, in order to ensure high-speed durability, the auxiliary belt layer has a two layer structure with a first layer wound around the main belt layer over a full width thereof and a second cord layer is wound on the first layer at regions corresponding to island portions of the tread. Also, the auxiliary belt layer has such a structure that a ribbon-shaped ply obtained by arranging plural cords side by side and coating them with rubber is helically wound around the main belt layer over a full width thereof to uniformly overlap the wound ribbon-shaped plies with each other at a given amount in widthwise direction.
In the above first defined structure of the auxiliary belt layer, the operation of winding the second layer on the first layer at regions corresponding to the island portions becomes complicated. Furthermore, since the cord end of the second layer is not restrained at each region, the hoop effect in the circumferential direction of the tire is low and also the problems such as tread separation and chunk-out are caused in the vicinity of the cord end. As a result the high-speed durability corresponding to the amount of the belt reinforcing material used. Moreover, the starting end and terminal end of the cord are existent in each of the regions, so that there is caused a stepwise difference in the rigidity of the carcass, which badly affects the uniformity of the tire.
In the structure of the auxiliary belt layer, the uniformly overlapped region of the ribbon-shaped ply is existent in the widthwise direction, so that it particularly tends to make the gauge of skid rubber located beneath the groove thin (not more than 1 mm). On the other hand, the skid rubber gauge is required to be 1-2 mm at minimum in order to make the thickness of tread rubber itself thin from viewpoint of high-speed durability in high-speed running tires and to ensure the depth of the circumferential groove from a viewpoint of drainage property. As described above, however, the skid rubber gauge is too thin in this structure of the auxiliary belt layer, so that there is a problem of creating cracks at the bottom of the circumferential groove during high-speed running.